1. Field of the Invention
This invention relates to a high-speed CMOS output buffer and in particular to a high speed CMOS output buffer with a controlled slew rate.
2. Description of the Prior Art
CMOS buffers are used to drive the input of CMOS circuits directly or through a bus line. The buffers function as an isolating circuit that prevents a driven circuit from influencing the driving circuit. The signal output of the buffer needs to be free of noise signal, cross-talk, and any undesirable interfering signals.
Prior art CMOS output buffers are known to exhibit very fast output voltage transitions. In an integrated circuit, this rapid rate of change of the output voltage (dV.sub.o /dt) of a CMOS output buffer causes large power supply current surges, cross-talk, i.e., undesired energy appearing in one signal path as a result of coupling from other signal paths, ringing, and other undesirable system effects. These effects are magnified in high pin count packages, i.e., packages with more than 100 output pins.
In normal application, there is always a capacitive load C, as shown in FIG. 1 for example. Transient current surges occur during periods when the output voltage is rising and falling. The larger the capacitor, the greater is the noise problem. Capacitors of 100-300 picoFarads are frequently used in CMOS buffer circuits and surge currents occur with resultant increased noise effects.
For example, in U.S. Pat. No. 3,631,528 a buffer circuit is described wherein a high capacitive load is driven, the circuit dissipates very little power internally, but the power dissipation contributed by the capacitive load is substantial. The inverter stages of the patented buffer circuit are small so that current and power used in those stages are reduced during switching. The buffer circuit has rise and fall times that are relatively fast, and the surge current is very high. The high surge current that appears at the output of the load capacitor generates undesirable noise. The larger the capacitor, the higher is the current surge and as a result there is increased noise signal.